This invention relates to freeing ash and sulfur in the form of clay and pyrite particles implanted on the surface of coal particles, particularly, ultrafine coal particles, for dispersion as a colloid in a colloidal suspension and then effecting a separation of the impurities from the coal particles. More particularly, the present invention provides a process to mechanically and/or chemically remove clay and pyrite from raw coal or a run of the mine coal mixture by treating coal particles with high energy chemical and/or physical forces such as those generated by oxygen in its several forms such as ozone or by oxidants, or by ultrasonics, or by each in the various possible combinations to release clay and pyrite particles that are adhered to the surfaces of the coal particles by mechanical or chemical bonding or by an electrostatic force as well as releasing such impurities from a newly-exposed surface of subdivided coal particles.
Coal-cleaning processes according to the present state-of-the-art are generally ineffective for removing ash and sulfur constituents wholly embedded within or bonded to the exposed surfaces of coal particles. In these coal-cleaning processes, impurities in a free state such as clay, shale, sand, pyrite and other minerals are selectively separated from coal particles using devices that depend upon relative differences in specific gravity of the particles to be separated, such as with dense media circuits or the relationship of particle size to bulk density, such as in up-current classification or the differential affinity of coal for other hydrocarbons, such as in froth-flotation and oil agglomeration.
As the particle size of coal and associated impurities in a free state diminishes, the separation process becomes more and more difficult and is executed with less and less efficiency. Since ultrafine impurities, particularly clay particles, are hydrophilic and thus display a great affinity for water, the recovery of ultrafine coal of less than total purity bears the additional burden imposed by the need to reduce moisture content through expensive means such as thermal drying. As a result, historically, it has been the practice to waste the ultrafine coal particles with their associated impurities to ponds or dumps or accept the practice of recovering an impure product with high moisture content and low market value.
The economics for the combustion of coal as fuel place a premium on coal with a low ash and sulfur content. Non-combustible ash, such as clay, sand, shale and other minerals require collection, material handling and disposal; whereas sulfur components, regardless of the form of occurrence, must be effectively removed from the combustion gases with other particulates including fly ash.
It has been proposed to introduce ozone into a vessel containing raw coal in an aqueous suspension to modify the surface of the coal particle and release the ash constituents, thereby permitting a more thorough separation of coal from ash by conventional means. The ozone is introduced into a vessel containing a slurry of coal crushed to a size range of 1/2" or less in a highly-agitated state. Ozone is pumped into the bottom of the coal-water mixture at the rate of 1/10 to 5 pounds per ton of coal and preferably about 1/2 to 1 pound per ton of coal. The reaction time is from 5 to 30 minutes at room temperature and atmospheric pressure. Thereafter, a series of separation steps are employed to separate the coal from the ash. First, the coarser particles are screened from the ultrafines that are then fed through cyclone or Deister table separators. Underflow from the separating screen, consisting of fine coal and ash, is to be treated in froth-flotation cells. Since the effect of ozone treatment on the surface of the fine coal particles is one that destroys the ability of the coal particles to properly respond to froth-flotation treatment, the recovered product, if any can be recovered, will still contain the majority of the impurities and, in particular, flakes of pyrite less than 25 microns in size. In such a process, a substantial percentage of the recoverable carbon is lost with the tailings that exit from the froth cell. The process does not attempt a separation of impurities from coal particles less than 105 microns and uses conventional devices for dewatering or moisture reduction which is inadequate. Further, since the ozone or oxidant can only react to impurities exposed on the surfaces of the coal particles, the ratio of exposed surface area to weight of individual particles renders the treatment of little practical value in the reduction of ash and sulfur in particles of a size greater than 30 mesh or 595 microns. Moreover, the cost of the oxidizing process is increased by the consumption of ozone or oxidant by the impurities, particularly pyrite, in a free state in the raw coal slurry, viz-a-viz those bonded to the surface of the coal particles and targets for the treatment. It has been discovered, contrary to the known belief, that oxidizing coal in a slurry with ozone is a highly pH dependent process. Further, the most favorable environment for efficient gas-to-liquid-to-solid contact in the process to treat coal with ozone in a slurry is under quiescent conditions. Also, contrary to the known belief, after the water slurry of coal particles is treated with ozone, the oxidized surfaces of the coal particles are so altered that separation of tailings from the coal particles cannot be carried out by conventional means such as froth-flotation. The economic impact of a substantial loss of carbon in the tailings could render the entire process uneconomical, if not unworkable, in many possible applications. By actual tests, it has been proven that all but a minor portion of the ash and sulfur reduction attributed to the ozone treatment, according to the known process, can be achieved through a careful application of conventional means.
In U.S. Pat. No. 4,328,002, there is disclosed a process for treating coal to remove sulfur and ash which involves the steps of preconditioning coal particles in the presence of an aqueous solution of an oxidizing agent, e.g., detergent; washing the pretreated coal with water; contacting the coal with an aqueous solution of an oxidizing agent until an exothermic reaction between the coal and oxidizing agent peaks and the pH drops to the range of 2 to 3; removing the coal from the oxidizing agent; contacting the coal with a passivating agent until the temperature of the coal drops and the pH rises into the range of 3 to 5; neutralizing the coal up to a pH of about 9; washing the coal with water and drying the coal. It has been discovered that the use of the disclosed oxidants H.sub.2 O.sub.2, HNO.sub.3, HCLO.sub.4, HF, O.sub.2, air and mild NH.sub.3 or CO.sub.2 is substantially ineffective to provide a useful result. Moreover, the known process treats the raw coal feedstock with the oxidants without first separating free impurities and sulfur which consume reactants and impede the recovery of ultrafine carbon by conventional means. Conventional means, such as froth-flotation, cannot be used to separate ultrafine impurities that are freed by the action of the oxidant from the clean carbon particles. The known process for using an oxidant for coal, as just described, is further deficient because it does not provide for the separation of impurities from coal particles less than 105 microns in size which also does not permit adequate dewatering or moisture reduction of the coal.
In U.S. Pat. No. 4,326,855, there is disclosed a process for beneficiating and stabilizing coal. A water slurry of coarsely-ground coal is agitated and then separated by froth-flotation. The tailings are ash and pyrite. Coal and water recovered from the froth-flotation process are fed to an attrition mill where the coal is ground and fed to a sonic reactor together with the optional addition of oil. The slurry is sonified through violent sonic agitation with sufficient energy to cause cavitation and for a time sufficient to reduce the particle size of the coal even further because pyrite and ash break away from the coal. The agglomerate and water mixture is then screened to separate coal particles from most of the contaminated water. The recovered coal-water mixture is about 10% to 40% water. The spherical agglomerates are mixed with oil to about 0.6 times the weight of the coal to produce a fuel. It is undesirable to form an agglomerate through the use of oil since this impedes separation of tailings from the treated coal particles. The present invention provides a process to overcome the shortcomings and disadvantages of known processes for beneficiating coal.